1. Field of the Invention
This invention relates to integrated circuit fabrication, and more particularly, to an enclosure for valves, fittings and other devices which may be associated with chemical gas/liquid distribution lines found generally in a piping industry such as, for example, semiconductor fabrication.
2. Description of the Relevant Art
Integrated circuit fabrication is a complex process involving many steps. To form a metal-oxide-semiconductor (MOS) integrated circuit, for example, a gate dielectric, typically formed from silicon dioxide ("oxide"), is formed on a semiconductor substrate which is doped with either n-type or p-type impurities. For each MOS field effect transistor (MOSFET) being formed, a gate conductor is formed over the gate dielectric, and dopant impurities are introduced into the substrate to form a source and drain. Such transistors are connected to each other and to terminals of the completed integrated circuit using conductive interconnect lines.
Process steps such as those described above often require the use of liquid or gaseous chemicals. For example, the gate conductor formation described above typically involves deposition of a polycrystalline silicon ("polysilicon") layer using silane gas. The gate conductor may be etched from this polysilicon layer using a dry etch process involving chlorine and/or fluorine-containing gases. The chemicals needed are typically distributed using pipes or tubing between the corresponding sources (such as bulk gas, gas cylinders or liquid chemical tanks) to the tools (such as deposition or etching reactors) which use the chemicals.
This distribution of chemicals is subject to various constraints. For example, purity of the chemicals should be maintained so that contamination of the fabricated circuits is avoided. Another important constraint is that the chemicals be distributed according to proper safety procedures. Some of the chemicals used in semiconductor fabrication may be toxic, corrosive, and/or pyrophoric (prone to combustion when in contact with air). Such chemicals may therefore be subject to specific distribution requirements. For example, double containment is typically used for toxic, corrosive, or pyrophoric chemicals. Double containment refers to the use of an inner container or conduit containing the chemical to be distributed, surrounded by an outer container or conduit which is connected to an exhaust system. In this way, any gaseous chemical which is leaked from the inner container or conduit is drawn into the exhaust system rather than escaping into the ambient of the fabrication facility. The exhaust system may include a scrubber or other means of reducing any hazards associated with the exhaust gas. For double containment of liquid chemicals, the outer container or conduit typically includes a drain in addition to an exhaust connection, so that any liquid which is leaked from the inner container or conduit moves into the drain system of the fabrication facility.
The double containment described above may be implemented in various ways. For example, a chemical source such as a gas cylinder is typically enclosed in a cabinet which is connected to an exhaust system. The chemical may be delivered from the cabinet to the tool using a double-walled conduit, such as a tube or pipe, having a space between the inner and outer walls. The chemical is transported within the inner wall of the tube, while the space within the outer wall is connected to an exhaust system (and a drain, in the case of a liquid chemical). Measurement and/or control devices are generally installed somewhere along the path between the chemical source and the tool. Examples of such devices include valves, pressure gauges, pressure regulators, and filters. Because these measurement or control devices control or sample the chemical being transported through the conduit, the outer wall of the conduit is removed at the installation point, and the device is installed between portions of the inner wall. Installation of a measurement or control device therefore disrupts the double containment of the double-walled conduit. Such devices, and the portions of the conduit in which they are installed, must therefore be surrounded by an enclosure if double containment of the system is to be maintained.
An example of an enclosure which may be used to provide double containment in the vicinity of measurement and/or control devices installed along a gas conduit is shown in FIG. 1. Conduit 10, including outer conduit 12 and inner conduit 14, is connected to either side of enclosure 16 (the connector used is not shown in FIG. 1). In the embodiment of FIG. 1, enclosure 16 includes box 18 and door 20. Outer conduit 12 is connected to box 18 at wall 19. Inner conduit 14 extends through a hole such as hole 28 in wall 19 into box 18, where its path is interrupted by one or more installed devices such as valve 22 and pressure gauge 24. External to enclosure 16, inner conduit 14 is surrounded by outer conduit 12. Enclosure 16 surrounds devices 22 and 24 and a portion of inner conduit 14 unsheathed from outer conduit 12. The volume between conduits 12 and 14 and the volume within enclosure 16 combine to form a containment volume which surrounds inner conduit 14 and through which the ambient gas (typically air) flows to an exhaust system through one or more exhaust connections 30. Door 20 of enclosure 16 provides access to devices 22 and 24.
There are some problems associated with the use of enclosures such as enclosure 16 of FIG. 1, however. For example, inner conduit 14 of FIG. 1 must be threaded through holes 28 in either side of box 18 before devices 22 and 24 can be installed. Installation of and access to devices such as 22 and 24 must therefore be accomplished with the devices inside the enclosure. Such measurement and control devices must be accessed for many reasons, including routine maintenance (such as changing filter cartridges), repair of faulty devices, and reconfiguration or upgrading of the chemical distribution system (e.g., for installation of new or replacement tools). The requirement that such maintenance, repair, or reconfiguration must be performed with the devices inside of the enclosure can cause problems in the fabrication facility. The designer of the distribution system is often forced to choose between undesirable options, such as making the enclosure oversized to allow adequate working room within it, or having to discard and replace some sections of conduit in the event that repair or reconfiguration is required, because there is insufficient room to properly disassemble or repair components within the enclosure. Making the enclosure oversized is undesirable because this takes up extra space within the fabrication facility. Fabrication facility space is limited and costly, and space used up for chemical distribution is space made unavailable for other functions, such as placement of processing tools. If the enclosure is made more compact to save space, however, multiple devices and/or multiple conduits may be spaced so closely together that a particular device cannot be accessed. In this case, an entire conduit, and possibly adjacent conduits, may have to be cut, removed and replaced in order to perform a repair on one device. In addition to the associated direct costs, such conduit replacement may result in otherwise unnecessary tool downtime.
It would therefore be desirable to develop a means of providing containment of measurement and control devices installed in a chemical distribution system, while allowing improved access to the installed devices.